DarkSide-50: The Hunt for the Elusive Dark Matter Gary W. Forster Experimental Gravitational and Particle Astrophysics Group at The University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA. OPTIONAL LOGO HERE. OPTIONAL LOGO HERE. How do we find it?. What is Dark Matter?.

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Experimental Gravitational and Particle Astrophysics Group at The University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA

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How do we find it?

What is Dark Matter?

Dark matter is just that: dark. It is called this because it doesn’t interact with the E&M spectrum (i.e. light) like normal matter we are used to does, and therefore does not reflect light. One of the reasons we care so much about dark matter is because 96% of the mass in the universe today is made up of dark matter and dark energy. There are different theorized forms of dark matter, but the one that the Darkside-50 project aims to find is called “WIMPS,” or Weakly Interacting Massive Particles.[1]

The details for the design of the Darkside-50 (50 kg Depleted Argon Cryogenic Scintillation and Ionization Detection) dark matter detector are almost official. From the outermost layer, the detector contains a water tank, a scintillator vessel, and a dewar.

When the electron enters the gaseous argon, it gives off more light. This phenomenon is similar to a neon sign emitting light. The signal from these photons is called S2.

A histogram produced by Root showing the intensities of gammas produced by decays in the U238 decay chain.

Another source of background is radiation from space. Namely cosmogenic neutrons and muons. To escape these unwanted highly energetic particles, Darkside-50s detector site at Gran Sasso National Laboratory is located under 1400 meters of rock.

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Determining What’s Useful

Gravitational Lensing – The theory of relativity predicts that the path of a light beam passing by a source of gravity will be bent by the gravitational field. Galaxies are bending light to a far greater degree than we thought they would, indicating that they are much more massive than previously hypothesized.[2]

A highly representative depiction of the common phenomenon of gravitational lensing.[3]

(Image has been altered from it’s original form)

Galactic Rotation – The speed at which objects rotate around the center of a galaxy is expected to decrease as the radial distance between the two objects increases. It was observed in 1930 by Fritz Zwicky that the speed actually did not decrease with an increase in radial distance. This can be explained by the existence of dark matter around the edges of a galaxy.[4]

Why Do We Think It Exists?

The detectors PMTs are sensitive to very small amounts of light, which will not usually be produced by a WIMP colliding with argon. Darkside-50 strives to filter out as much background signal as possible. One way to tell what is a useful signal and what is background is by looking more closely at S1 and S2, and specifically by integrating the signal over time. WIMPS, for example, should deposit relatively high amounts of their energy in a short amount of time in what is called a ‘recoil event,’ resulting in a high light yield quickly. This signal can be easily identified as a particular type of event and distinguished from other events with a different integrated S1 signature. The integrated S2 will also look different depending on what causes the event. The integration of S1 and S2 play a large role in determining what is a useful signal, and what is extraneous and unwanted background.

Geant4 Simulations

Because the detector relies so heavily on signals from photons to collect data, it is important to accurately reproduce real-life optical processes within the simulation. Production of the code which calls upon Geant4’s internalized optical processes is still in the works, but when it is completed, the simulations will be able to handle phenomena such as Rayleigh scattering, Fresnel and Lambertian Reflection, and Cerenkov light. The image below was produced through code fed to Geant4 and viewed using 3D imaging software.

A computer-generated image of the inside of the simulated scintillator vessel. In this picture, much of the outer portion of the detector is made invisible to allow you to see past it.

The spherical scintillator vessel, and cylindrical dewar containing gaseous and liquid argon. The golden bulbs on the inside of the scintillator vessel and below/above the liquid argon (yellow) PMTs (photomultiplier tubes) and are used to detect small amounts of light.[6]

When a WIMP enters the pool of liquid argon, it can hit an atom of argon. When this interaction occurs, photons are produced and are shot in all directions. The signal from these photons is called “S1.”

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Another thing that happens during the collision is that electrons get knocked off the argon. Since argon is a noble gas, other argon atoms don’t want to pick up the loose electrons. An electric field accelerates the electrons into the pocket of liquid argon.

Because the detector is sensitive to very small signals, very careful analysis is put into every detail of the detector. An example of the attention to detail is in the analysis of the decay chain of uranium 238. While uranium isn’t used in the detector, it may be present in some of the materials that are, no matter how pure they might be. While the simulation has a model for how U238 decays, it is still necessary to take a close look and analyze every detail so that we aren’t shocked or thrown off by what we see.

Ultra-Low Background

References

Galactic rotations as predicted classically (A) and seen through observation (B).[5] This discrepency offers support for a halo of dark mass near the edge of galaxies.